An estimation based on field measurements of water ?ow from 69 rivers has shown that the Mediterranean drainage basin (some 4,135,000km 2) has the ability to provide 570 km (0.23 myr-1) of freshwater. During the second part of the 19 ^ century the construction of thousands of small and big dams has reduced this natural freshwater ?ow almost to the half. This reduced value corresponds to 0.11 myr- , which is comparable to the Black Sea net out?ow .